One area of interest for improving the speed and performance of semiconductor devices includes strained silicon technology, which has been shown to enhance carrier mobility in both n-channel and p-channel devices, and is being considered to improve the electron mobility and drift velocity in n-channel MOSFETs in CMOS technology.
There has been considerable research using strained silicon germanium layers on silicon to increase the hole mobility of p-channel CMOS transistors. Thin layers of silicon germanium have been fabricated on silicon because of the tolerance of the thin silicon germanium layers to strain. FIG. 1 illustrates a relationship between elastic strain and semiconductor layer thickness, and indicates that thin layers of silicon germanium on silicon are more tolerant of strain than thick bulk samples. The semiconductor yield is plotted with respect to plastic deformation and defects in bulk samples.
Solid phase epitaxial (SPE) regrowth of silicon on sapphire is known. The SPE regrowth of silicon reduces defects and dislocations that occur during the initial epitaxial deposition of silicon on sapphire because of a large lattice mismatch. A silicon implant amorphizes the initial silicon layer, and regrowth is accomplished at a low temperature. Subsequently, strained layers of silicon germanium have been grown on silicon by SPE.
Silicon germanium layers have been grown on silicon by ion implantation and regrowth by laser melting. Silicon germanium layers have also been formed by ion implantation and regrowth by SPE. The use of ion implantation to form silicon germanium layers, and the use of SPE to regrow layers of silicon germanium on silicon have been described separately and in combination with one another.
Thin layers of strained silicon are being considered for CMOS n-channel devices. Thinner layers of silicon are more tolerant of strain. One technique for producing strained silicon involves epitaxially growing the silicon and silicon germanium layers using an ultra-high vacuum chemical vapor deposition (UHVCVD) process, a costly and complex process, to form silicon layers on relaxed silicon germanium layers. A large mismatch in the cell structure causes a pseudo-morphic layer of silicon on relaxed silicon germanium to be under biaxial tensile strain. The biaxial strain modifies the band structure and enhances carrier transport in the silicon layer.
The strain on the silicon layer depends of the lattice constant difference between silicon and silicon germanium. The lattice constant of silicon germanium is between the lattice constant of silicon (5.43095 Å) and the lattice constant of germanium (5.64613 Å), and depends on the percentage of germanium in the silicon germanium layer. FIG. 2 illustrates the lattice constant of a silicon germanium (Si1-XGeX) substrate for different percentages (X) of germanium. FIG. 3 illustrates the mobility enhancement for strained silicon for different percentages (X) of germanium in a silicon germanium (Si1-XGeX) substrate. The mobility enhancement increases as the percentage of germanium increases, and levels off to around 1.6 when the percentage of germanium is around 22% or larger.
There is a need in the art to provide improved methods of forming strained silicon on silicon germanium structures to improve the speed and performance of semiconductor devices such as CMOS n-channel devices.